"Misurata is free, the rebels have won. Of Gaddafi's forces, some are killed and others are running away," rebel spokesman Gemal Salem told Reuters news agency by telephone from the city.
Soldiers captured by rebel fighters on Saturday said the army had been ordered to retreat from the western port city.
"The rebels attacked us while we were withdrawing from Misurata near a bridge this morning," said Ayad Muhammad, a young soldier.
At least 10 people were killed in street battles in the city on Saturday, a doctor told AFP at a hospital overwhelmed by an influx of casualties including government soldiers.
"Since eight o'clock this morning, we have received 10 dead and 50 wounded, which is usually the number for a full day," Khalid Abu Salra said at the main Hikma hospital in Libya's third-largest city.
"We're overwhelmed, overwhelmed. We lack everything: personnel, equipment and medicines."
Hundreds of civilians have been killed in the fighting in Libya's third-largest city.
Saturday's upsurge in the battle for Misurata comes after the Gaddafi government said its soldier had been issued an "ultimatum" by local tribes to stop the rebellion in the city, 200 kilometres east of the capital Tripoli.
Libyan officials late on Friday acknowledged that the siege had been broken when rebels seized the port and NATO air strikes had taken their toll.
"The tactic of the Libyan army is to have a surgical solution, but it doesn't work, with the air strikes it
doesn't work," Khaled Kaim, the Libyan deputy foreign minister, said in the capital, Tripoli.
"The situation in Misurata will be eased, will be dealt with by the tribes around Misrata and the rest of Misurata's people and not by the Libyan army."